Balancing traffic over multiple links in a nssa ospf area

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Aug 16th, 2007
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I?m finding difficulties balancing traffic over multiple links between an Hybrid (Catos/IOS) Cisco Cat 6513 and a Cisco 12000 router managed by my ISP. There are 3 gigabit links between the two devices and I have configured three subnets /30 mapped with three different vlans. All subnets are in the same nssa area in my ospf . Now I correctly see three neighbors on the three vlans but in both routing table and ospf database I only see one ip route for each destination (always the one with higher ip address). Does anyone know something that can help me?

Thank you

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Anonymous (not verified) Wed, 08/22/2007 - 11:05
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Redistribution into an NSSA area creates a special type of link-state advertisement (LSA) known as type 7, which can only exist in an NSSA area. An NSSA autonomous system boundary router (ASBR) generates this LSA and an NSSA area border router (ABR) translates it into a type 5 LSA, which gets propagated into the OSPF domain.

royalblues Wed, 08/22/2007 - 11:39
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Is the cost associated with each link different?


moitessier Thu, 08/23/2007 - 01:04
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The Costs associated with every single link are identical. I?m not sure but It seems like nssa areas do not allow to balance over multiple links. I wonder why...

Best regards

moitessier Thu, 08/23/2007 - 01:11
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Thanks for the explanation, do you think this can impact on the use of multiple routes for the same destination by the routers connected to the same nssa area?

Best regards

karim.benhabyles Wed, 09/23/2009 - 09:05
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Hi all,

I have the same issue in my network.

Did anyone had an idea why this happen?



Giuseppe Larosa Wed, 09/23/2009 - 10:54
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Hello Karim,

let's suppose you have an NSSA area with two ABRs.

Let's suppose that ABR1 is the only one that translates the LSA type 7 NSSA external route to an OSPF type 5 LSA and to inject it on the area 0.

We would expect ABR2 that connects area 0 and NSSA area and that receives the LSA type 7 to generate its own type 5 LSA.

Actually, ABR2 is suppressing its own advertisement because it receives on area0 the LSA type5 generated by ABR1.

The choice should be based on OSPF router-id.

the end result is that from outside the NSSA area all traffic is going through ABR1 only.


yes highest OSPF router-id NSSA ABR wins see

If bit P = 1, then the NSSA ABR must translate this type 7 LSA into a type 5 LSA. If there are multiple NSSA ABRs, the one with highest router ID.

Hope to help


karim.benhabyles Wed, 09/23/2009 - 11:43
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Hi Giuseppe,

My situation is not exactly like that.

I have one ABR with two links to one ASBR

R1 --- R2 ===== R3

R1 and R2 have one link in area 0.

R2 and R3 have two links between theme on area 1.

The only interfaces on R3 that participate in OSPF are the interfaces linked to R2.

I have a loopback in R3 that is redistribute in OSPF using "redistribute connected subnets" command.

in R2 I have the command "area 1 nssa default-information originate no-summry" set.

in R3 I have the command "area 1 nssa" set.

As expected R3 have only IA* default route in his routing table with two next hops (the two links to R2 th ABR)and equal cost.

In the other hand, in the ABR (R2) I have only one route (N2) in the routing table going to and I would like to have two to just have loadbalancing.

I've done a test, I created another loopback let say and announce it on ospf using "network area 1" command. In this situation, the ABR have the route to with two next hops (the two links) as well as a route to with two next hops so in this situation everything is ok.

I don't understand why I have to aanounce a network ( to my ospf process on area 1 to have two route on my ip route (ABR).

How annoucing this a network is solving the issue?

Best regards,



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